The present invention relates to radars in general, and more specifically, to a radar having a plurality of receive beams stacked in elevation and a target height measurement extractor especially for use therein.
Typical examples of stacked beam radars including a target height extractor include the AN/TPS-43 manufactured by the Westinghouse Electric Corporation and another similar radar, the Martello radar manufactured by Marconi. An illustrative sketch of a stacked beam radar being used in a typical environment is shown in FIG. 1. The stacked beam radar includes an antenna assembly 20 for generating a multiplicity of receive beams 22 stacked in elevation. A radar unit, denoted by the block 24, is coupled to the antenna system 20 for operation thereof. The radar unit 24 may include a transmitter 26 for driving the antenna assembly 20 to transmit radar pulses at a pulse repetition frequency (PRF) and with a desired elevation coverage to illuminate targets, such as that shown at 28, for example. The interpulse periods of the pulsed transmissions constitute range sweeps of the radar. The radar is further operative to scan the pulsed transmissions and associated receive beams 22 in azimuth. Further included in the radar is a multiplicity of echo conditioning or receiving channels 32 respectively corresponding to the multiplicity of receive beams 22. The receiving channels 32 listen simultaneously on all of the beams 22 for echoes, generally of the same frequency. Target information 34 derived from the receive beam echoes by the receiving channels 32 may be provided to a three-dimensional (3D) computer 36 which functions to compute the range R, height H and azimuth .theta. measurements of a detected target.
In operation, the antenna 20 of the 3D radar may be scanned continuously in azimuth, either mechanically or electronically, and produces a burst of receive beam echoes of varying intensity as the multiple stacked beams pass through a target, like that shown at 28, for example. The receiving channels 32 function to discriminate between desired echoes from aircraft and other objects similar thereto and undesired interference found in the received echo signalling. The undesired interference may result from such environmental factors as rain, chaff, clutter from ground terrain 38 or slow-moving clouds 40, for example, and pulses from external sources and the like. The function of the 3D computer 36 is to combine the information 34 derived from these multiple receive beam echoes into the best estimate of the target position in three-dimensional space--range R, height H, and azimuth .theta..
A more specific example of a present 3D radar is shown by the block diagram schematic embodiment of FIG. 2. Each receive beam echo conditioning channel 1-n may include a receiver R1, R2, . . . Rn which is operative to reduce the radio frequency of the receive beam echoes to an intermediate frequency signal IF1, IF2, . . . IFn, respectively. Each echo conditioning channel of 32 further includes a target extractor section D1, D2, . . . Dn which in each case is coupled to the output of the corresponding receiver to extract target information from the IF signalling generated thereby. Each target extractor section Di may include a constant false alarm rate (CFAR) section and a linear or logarithmic decoding section. The CFAR section conventionally includes a CFAR decoder and detector 50 which provides output signalling 52 to two parallel paths, one including an integrator 54 and a first threshold circuit 56 coupled in cascade therewith and the other path including a second threshold circuit 58.
Generally, the integrator 54 integrates the output signalling 52 over the range sweeps corresponding to an azimuth scan of the radar beam across a target to generate an integrated output signal 60. In the threshold circuit 56, the integrated signal 60 may be compared to a threshold level, which is set above the noise level of the radar, to identify a target condition. Accordingly, the threshold circuits 56 of the multiplicity of echo conditioning channels generate respectively first signals F1, F2, . . . Fn which correspond to a time in each range sweep where a target is identified. Similarly, the output signalling 52 of each channel may be additionally compared to another threshold level in the corresponding threshold circuit 58 to generate in each case second signals S1, S2, . . . Sn. The generated second signals Si correspond to times in each range sweep where potential targets are identified.
The linear or logarithmic decoding function of a target extracter section Di may include a conventional logarithmic decoder and detector circuit 62 for extracting the amplitude of the IF received beam echo pulses in logarithmic form and generating third signals Ti representative thereof. For the case in which the signal generated by the circuit 62 is in unipolar analog form, the generated signal may be stretched in time by a stretcher circuit 64, say on the order of 0.5 microseconds, for example, to make the timing of the data sample thereof less critical. The first, second and third signals generated from the receive beam echo conditioning channels are exemplary of the information signals 34 which are provided to the 3D computer 36, an embodiment of which being provided by the remaining block diagram schematic of FIG. 2.
Referring to the remaining embodiment, a height measurement extractor comprises the adder and subtracter units 70 and 72 which, respectively, add and subtract pairs of the third signals generated from the multiplicity of receive beam echo conditioning channels to generate corresponding sum and difference signals 74 and 76, respectively. The pairs of third signals operated on by the adder 70 and subtracter 72 correspond to adjacent receive beams of the receive beam elevation stack. An example of this operation may be described in connection with the illustrative sketch of FIG. 3. In this example, echoes from a target X1 are received in the adjacent beams B1 and B2 of the elevation stack. Resulting third signals from the echo conditioning channels corresponding to the beams B1 and B2 are related respectively to the points on the antenna beam patterns designated at 80 and 82, respectively. The resulting third signals may be added in the adder 70 and subtracted in the subtracter 72 to form their sum and difference signals, respectively.
The resulting beam pair sum and difference signals 74 and 76 may be respectively provided to a beam pair select circuit 84 and a multiplexer MUX/analog-to-digital A/D converter circuit 86, respectively. An OR gate 88 may be included in the computer 36 to monitor the multiplicity of echo conditioning channels for the generation of a second signal and to generate a "GO" pulse over signal line 90 in response thereto. The present embodiment achieves this function by coupling each input of the OR gate 88 to a corresponding output of the threshold circuit 58 of each of the decoding sections D1-Dn. A generated "GO" pulse via line 90 renders the beam pair select circuit 84 and MUX/A/D circuit 86 operative during the range sweeps of the radar.
The crossover angles of the receive beams of the radar, denoted as .phi..sub.1, .phi..sub.2, .phi..sub.3 in FIG. 3, for example, are the elevation angles where the adjacent beams have an equal gain. Values representative of these crossover angles of the multiplicity of stacked beams of the radar may be stored in an angle store memory 92. A particular gain or slope, associated with each crossover angle value, may be stored in a slope store memory 94. In a typical operation, the circuit 84 may select a beam pair, generally by determining the largest of the sum signals 74 and generate an address code, representative of the selected beam pair, to be provided to the memories 92 and 94 over signal line 96 and to the circuit 86 over signal line 98. Concurrently, a corresponding beam pair difference signal is selected from the signals 76 by the circuit 86 in accordance with the address code via line 98. The selected difference signal may be digitized in 86 and provided to a height computer 100 utilizing the signal line 102. The sign bit of the digitized signal may be provided to the slope store 94 over signal line 104.
A crossover angle .phi. may be accessed from the angle store memory 92 in accordance with the address code 96; and accordingly, an angle slope A may be accessed from the slope store memory 94 in accordance with the address code 96 and polarity bit 104. The accessed signals .phi. and A are provided to a height computer 100 for a corresponding height measurement computation performed thereby. A zero range trigger signal and clock pulses generated from an appropriate clock source of the radar may be provided to the height computer 100 over signal line 106. In addition, signals representative of a function of the refraction index of the atmosphere B and the tilt of the antenna .phi..sub.T may also be provided to the height computer 100.
Typically, a target X1 may reflect echo signals which are received by the beams B1 and B2 of the radar as shown by the sketch of FIG. 3. The value associated with the crossover angle .phi..sub.1 is accessed from the angle store memory 92 to the height computer 100. The difference signal which may be represented by .DELTA.1 is applied to the height computer over signal line 102 and the polarity thereof is provided to the slope store memory 94 over signal line 104. From the polarity of the difference signal and the address code of the crossover angle, a slope A is selected from the memory 94 for use in the height computer 100. In the height computer 100, the selected gain factor A scales the difference signal or monopulse error .DELTA. to obtain the estimate of the target's angular deviation .DELTA..phi. from the crossover angle .phi..
For example, the monopulse difference .DELTA.1 for the target X1 has a positive polarity resulting in the selection of slope A1 for the computation of the angular deviation .DELTA..phi.1 (i.e. .DELTA..phi.1=A1.multidot..DELTA.1). Similarly, for another target, say X2, for example, the resulting difference signal .DELTA.2 may be used to access another slope A2 from the memory 94 which is used accordingly to calculate the angle deviation .DELTA..phi.2 associated therewith.
Accordingly, the height computer 100 computes height estimates for each individual echo corresponding to a generated "GO" pulse over signal line 90 within each range sweep of the radar. An example of an estimated height computation is provided by the following equations: EQU H=R sin (.phi.+.DELTA..phi.+.phi..sub.T)+BR.sup.2, (1)
and EQU .DELTA..phi.=A.DELTA., (2)
where:
H=height PA1 R=range PA1 .phi.=crossover angle of selected beam pair PA1 .DELTA.=difference in log data from adjacent beams of selected pair PA1 A=selected slope parameter, function of selected pair and polarity of .DELTA. PA1 .phi..sub.T =tilt of antenna PA1 B=function of index refraction.
The range R for each height computation may be obtained in a number of different ways, one way may be to use a conventional digital counter which is governed by the zero range trigger signal and clock pulses provided to the height computer 100 over signal lines 106. The waveforms 4A-4D illustrate this range computation operation. Each waveform of FIG. 4 is representative an exemplary range sweep of the radar. For example, if a GO pulse is generated in a range sweep as exemplified by the waveform 4A, a height computation will result shortly thereafter represented by the pulse interval of waveform 4B. Should a counter be used for range determination, the clock pulses shown by the waveform of 4C may be used to increment the counter having an output exemplified by the waveform of 4D. At the time of the height computation, the counter output may be sampled to acquire the range R' for use in the instant computation.
For any given target detected by the radar, there may be a succession of range sweeps in which GO pulses are generated, i.e., as the stacked beam passes through the target in azimuth. For each range sweep in which a GO pulse is generated, a range R' and a height H' is generated by the height computer 100 and provided to a height centroider 100 which combines the multiple range R' and height H' computations into a single range and height report, denoted as R and H, respectively. In the instant embodiment, this is accomplished in the height centroider 110 by storing the first N height estimates at the same range received from the height computer 100. Thereafter, each new height report received at the same range causes a comparison to be made between the (N+1) individual reports and the average of the N stored reports; the height report with the largest deviation from the average may be rejected, and the most consistent N may be retained in storage. While this operation minimizes the size of the storage of the centroider 110, at the same time it sacrifices some of the accuracy benefits of the extra estimated height data. For example, if it takes seven range sweeps to pass through a target in azimuth, may be only the information from three range sweeps may be used in the height centroiding process of unit 110. The centroided range R and height H messages are passed to a message associator 120 when no further reports are generated at the same range for M range sweeps.
Concurrent with the foregoing described operations, another OR gate 112 may be used to monitor the multiplicity of echo conditioning channels for the generation of a first signal in the range sweeps and to generate a signal over line 114 in response to that event. The event signals are provided to a conventional range and azimuth centroider 118 along with the zero range trigger and clock pulses via signal lines 116 to generate a range R and azimuth .theta. measurement for the detected targets independent of the height extractor operations. The range R and azimuth .theta. measurements of the centroider 118 are associated with the range R and height H measurements from the height extractor in the message associator circuit 120 to produce the range, azimuth and height measurements for a detected target. Generally, the association of messages is based on the proximity of times of the message outputs, approximate azimuth, and of their ranges.
While present radars of this variety have operated adequately to perform their desired functions, they perform these functions with a number of drawbacks. Referring to the graph of FIG. 5, in some implementations, the third signals generated by the multiplicity of echo conditioning channels are sampled from range sweep to range sweep at a time which is defined by the leading edge Gi of the generated GO pulse, which varies in time position (e.g. G1, G2, and G3) relative to the center of the target echo pulse as the echo amplitude thereof changes from range sweep to range sweep. With the use of phase discrimination type CFAR processing, this range uncertainty is controlled to within tolerable limits. Thus, the stretcher circuit 64 is capable of spreading the logarithmic signal in time sufficiently to cope with this limited uncertainty. However, amplitude discrimination CFAR processing may create much larger range uncertainty, making the required delay time of the stretcher 64 at times prohibitive.
Another drawback is in the functioning of the message associator 120 wherein the R and H measurements of the height extractor via centroider 110 and the R and .theta. measurements of the centroider 118 do not occur simultaneously and thus can occasionally fail to associate. Moreover, if there are two aircraft at nearly the same range and azimuth (or an aircraft and clutter or surface vehicle), the measured heights thereof by the centroider 110 may be incorrectly associated with the range and azimuth measurements of the centroider 118. Still another drawback is encountered under the conditions in which an erroneous height over clutter occurs even when the target aircraft is in a beam with insignificant ground clutter interference. Under these conditions, the beam selection logic of circuit 84 may select the lowest beam pair if its clutter creates a stronger echo signal (i.e., larger pair sum) than the actual target aircraft in a higher beam.
In addition, the monopulse difference pairs for estimating the elevation angle difference are approximated by two straight lines (one for positive data and the other for negative) defined by two slopes for each crossover angle. Because the actual difference curves are not linear, significant erros may be created by the present curve approximation. This error in general is largest in the upper beam pairs where the individual beams are covering a greater elevation extent. It is recognized that the height computations are most accurate when the stacked antenna beam is pointing directly at the target, that is, if the receive beam echoes therefrom do not exceed the dynamic range of the receiver. Here again, because of the varying shapes of the elevation beam patterns, the target echoes received on the sides of the beam patterns from receive beam to receive beam are more likely to be less accurate than those echoes received from the receive beam center because their signal-to-noise ratio is lower and because the monopulse difference curves are generally not identical to those near the beam center. Therefore, height computations involving simple averaging of multiple samples generally fails to provide proper emphasis to the stronger and more accurate receive beams echoes from the target.
Another compromise, which has been described hereabove in connection with the present height centroider 110, is that of the use of only a portion of the selected height computations from the height computer 100 to form the height message. Still further, under the operation of the present radars, a height computation is employed in the computer 100 to compute the height on each interpulse period for target aircraft which may be separated by only a few microseconds in range sweep. This imposes a constraint on the time to perform each height computation, thus forcing the use of a simple height equation (1) with a crude approximation BR.sup.2 used to compensate for the effects of atmosphere refraction which may at times introduce significant errors in the resulting height estimation. Refraction effects in this simple equation (1) are approximated by a change to B, equivalent to an effective increase in the earth's radius. Thus, the equation may only assume a constant bending rate at all points in space, and does not take into account the fact that rays at high altitude actually bend less than those at low altitude.